The present invention relates to a multiplexed data memory system in which an external memory control unit which receives a write command from a host unit and which memorizes the received write command in an external memory unit under its control as original data writes the same data as that original data in the external memory unit operable under different external memory control units as backup data. More particularly, the present invention relates to a multiplexed data reorganizing system capable of effectively utilizing backup data and which is also able to efficiently reorganize the duplexing of original data obtained after backup data was used and the backup data, and relates to a subsystem for carrying out this system.
In a multiplexed data memory system in which data used in a host unit such as a host computer is multiplexed and held by a plurality of external memory units, e.g. original external memory unit and sub-external memory unit, as a method of holding the multiplexing of data between the original external memory unit and the sub-external memory unit, there are generally known technologies disclosed in Japanese Published Unexamined Patent Application No. Hei 6-214853, Japanese Published Unexamined Patent Application No. Hei 7-6099, Japanese Patent No. 2509460 and Japanese Translation of PCT Application No. Hei 8-509565. According to the method disclosed in the above-mentioned technologies, an original external memory unit and sub-external memory unit are provided under control of different external memory control units, the original external memory control unit and the sub-external memory control unit are connected via an interface cable and a write command received at the original-side external memory control unit is issued to the sub-side external memory control unit, thereby resulting in the multiplexing of data of the original external memory unit and the sub-external memory unit being maintained.
In such multiplexed data memory system, as a method of reorganizing the multiplexing after the multiplexing is interrupted, there is known a method of copying only an update place of an original external memory unit which is interrupted in multiplexing to a sub-side external memory unit, i.e. so-called difference copy.
In this difference copy, data of the sub-side external memory unit is generally used as backup data of the original-side external memory unit. During data is duplexed, a write access and a read access are generally inhibited from a standpoint of coincidence of original data and sub-data.